Revise the translation for Korean (#450)

* Run msgmerge --no-wrap --update po/ko.po po/messages.pot

* Be consistent with the line ending. For example, if the original text
ends with ":", so the translation is.

* Remove unnecessary newline characters

If a newline character is from the rendering of the source text, remove
it in the translated text.

* Fix the remaining inconsistencies on line ending chracters

* s/제너릭/제네릭/g

제네릭 seems to be the popular translation for "generic":

제너릭: 120K results
제네릭: 1M results

* s/트레이트/트레잇/g

트레잇 seems to be the popular translation for "trait" at least in the
Rust ecosystem. The Rust book also uses 트레잇.
https://rinthel.github.io/rust-lang-book-ko/appendix-08-glossary.html

* Revive the missing translations for the Translations section

* Resolve remaining fuzzy translations

Author: jiyongp